The present invention relates to a plasma source for the surface treatment and/or coating of large substrates. More particularly the present invention relates to a linear plasma source for plasma enhanced chemical vapor deposition and for plasma surface treatment, in particular a plasma source based on a hollow cathode type discharge.
Various plasma sources are disclosed in the prior art for the deposition of thin films and chemical modification of surfaces.
When large substrates are to be treated, these plasma sources typically are linear ionic sources like the one disclosed by Madocks in U.S. Pat. No. 7,411,352. This plasma source is based on a magnetron discharge and produces a linear beam of ions or, by combining several sources, multiple parallel beams of ions directed towards the substrate surface. Madocks discloses that for coating purposes a coating precursor can be provided outside of the plasma sources. The plasma extends essentially only along one dimension, i.e. the length of plasma source. The width of the ion beam is limited by the pressure inside the process chamber which restricts the mean free path length. When a substrate is conveyed beneath this plasma source the contact time is therefore relatively short. Thus the number of plasma sources will have to be multiplied when for example the treatment duration of the substrate with the plasma needs to be increased. Furthermore a coating precursor injected next to the plasma source has limited opportunity to interact with the plasma beam. This results in relatively low deposition yield and increases the risk of soiling the coater with precursor that has not been able to react with the substrate surface.
Madocks also discloses that sputtering of the electrode material occurs and that the sputtered material redeposits and thus remains within the source. Sputtering of the electrode material however results in reduced lifetime of the electrodes. The redeposition of the sputtered material may also lead to blockage of the nozzles of the plasma source, making a uniform substrate treatment or coating impossible. Furthermore, the sputtered electrode material may condense and/or react further, leading to the formation of debris that either block the source's nozzles or fall on the substrate creating defects. These nozzles are made up by one of the plasma source's electrodes. The electrode is thus exposed to the coating process atmosphere in the vacuum chamber and therefore prone to soiling from the injected coating precursor.
Furthermore the magnetron discharge based source disclosed by Madocks requires magnets. Magnets are sensitive to high temperatures, these sources therefore cannot be run at high temperatures and need to be cooled by active or passive means. The presence of these magnets as well as the necessary presence of shunts lead to a complex and thus expensive assembly.
This source also produces a relatively low density of free electrons compared to plasma sources based on hollow cathode discharge. For coating purposes, the electrons of the plasma serve to ionize the coating precursor. Therefore the coating efficiency is low when a magnetron plasma based source such as disclosed by Madocks is used.
Jung discloses in EP0727508 A1 a hollow cathode linear plasma source based on two parallel electrodes. The plasma extends essentially only along one dimension, i.e. the length of plasma source, forming a narrow plasma beam. Jung discloses that in order to avoid sputtering of the electrode material a stream of inert gas has to be injected parallel to the electrodes. The injection of an inert gas parallel to the electrodes however leads to a reduction of the yield of reactive ions and therefore reduced treatment efficiency or coating yield.
One of the major problem with these kinds of high deposition rate sources is the fact that the walls of the plasma sources become quickly soiled by reacting prematurely with precursors that flow through the plasma. Due to this problem, the use of this process in industry is quite limited and requires frequent cleaning cycle which limits the throughput of the production line.
Another drawback of these high deposition rate sources is that it is difficult to confine the precursor to the substrate surface after it leaves the plasma source. As a consequence a significant portion of the precursor cannot be used to form a coating on the substrate. This leads to reduced coating yield and to soiling of the coater with the transformation of the precursor on surfaces surrounding the plasma source.
Thus there remains a need in the large area surface treatment and large area coating field for a simple plasma source that can provide a uniform plasma of considerable length that can treat and/or coat large substrates with high efficiency and low amount of soiling and of defects.